1. Field
The present disclosure relates to a control apparatus for an automatic transmission and a control method for an automatic transmission.
2. Description of the Related Art
Automatic transmissions that automatically perform shifting by switching power transmission paths each composed of a plurality of shift gear trains through selective engagement of frictional engaging elements, such as shift clutches, have been widely employed for vehicles. In recent automatic transmissions, in order to increase the power of engines and the shifting performance of the automatic transmissions, for example, shift operation (engagement of frictional engaging elements) of the automatic transmissions tends to be frequently performed. Accordingly, the automatic transmissions tend to suffer from an increased calorific value in engaging the frictional engaging elements during shifting. The increased calorific value of the frictional engaging elements might cause seizing (burning) of the frictional engaging elements and other problems. Thus, there is a need for protection control for protecting the frictional engaging elements against thermal damage.
Regarding such protection control, Japanese Patents Nos. 4913535 and 4913848 describe control devices for automatic transmissions. In the control device described in Japanese Patent No. 4913535, the temperature of at least one of frictional engaging elements to be engaged in shifting is computed. In a case where the computed temperature is higher than a reference temperature, if a shift operation to a target gear position is an upshift operation using the frictional engaging element to be engaged, the time of the upshift operation is delayed by a predetermined time. On the other hand, if the shift operation to the target gear position is a downshift operation, the downshift operation is allowed without a delay.
In the control device described in Japanese Patent No. 4913848, the temperature of a frictional engaging element is calculated, and the rotational difference between an input and an output of the frictional engaging element is calculated. Based on the calculated temperature and rotational difference of the frictional engaging element and a pattern of a speed change instruction, it is determined whether the frictional engagement element is engageable or non-engageable. If there is a possibility of burning of the frictional engaging element, the frictional engagement element is determined to be non-engageable, and engagement thereof is delayed until the frictional engagement element is determined to be engageable. In this manner, an appropriate cooling period is selected, and thus, engagement can be controlled with protection of the frictional engaging element against thermal damage.